The first electronic musical instrument was invented in 1874 by American Elisha Gray (1835–1901).
In early 1874, Gray saw his young nephew plugging several electrical circuits containing batteries and a vibrating metal strip into a bathtub, causing an audible hum as the circuits were opened and closed. In fact, Gray's nephew accidentally devised a primitive method of transmitting electrical vibrations produced by the spring-loaded metal strap.
Gray repeated the experiment in his laboratory and began researching a number of practical applications of this effect. One was the invention of a small keyboard instrument with enough single-tone "transmitters" to play an octave. He called it the “musical telegraph" and took the instrument with him on a concert tour to England in August and September 1874. At the end of the same year, he also produced a two-octave version. The instrument was polyphonic and was predating the first electric organ by sixty years.
The "Singing Arc" invented by Englishman William Duddell became a famous novelty at the turn of the 19th and 20th centuries. Duddell tried to figure out a way to eliminate the high-pitched sound emitted by the carbon arc streetlights. In 1899, he realized that he could actually control those penetrating tones through a secondary circuit connected to an arc's direct current, to modulate and control the oscillations of the arc. Later Duddell had a keyboard added to his instrument. It was an example of a device with very simple voltage-controlled frequency modulation.